SIXTH ORDINARY MEETING. 49 
The heat generated by the sun in assuming its present density and 
conformation can not be supposed to be greater than that produced 
by contraction from the limits of the solar system to a homogeneous 
sphere of one-half its present radius. 
This would make 44,000,000 years, the limiting age which can be 
assigned to the Cambrian formations. 
We shall conclude by applying the principle that the absorptive 
power of a vapour is determined by that of the liquid from which it 
is derived, to explain the empirical law (enunciated by Mr. McGee) 
that any increase in annual or diurnal range is accompanied by a 
diminution of mean temperature. The aqueous vapour of the 
atmosphere, being derived from water, which has a comparatively 
high absorptive power, must also possess considerable power of 
absorption, and Tyndall has conclusively shown experimentally that 
such is the case. Also, the power of radiation is strictly propor- 
tional to the power of absorption, as is known both from theory and 
experiment, so that the aqueous vapour of the atmosphere is both a 
good absorbent and radiant. 
Now, when the temperature is raised, not only will the aqueous 
vapour of the atmosphere be heated, but a larger amount of it will be 
formed, and as gases expand when heated, this vapour will tend to 
rise to the higher regions of the atmosphere, and radiate its heat 
into space. On the other hand, in a comparatively cold season the 
relatively cooled vapour tends to descend, the heated vapour from the 
surface of the earth ascends, and imparts its heat to cold space. 
Also the amount of heat received from the sun may for our 
present purpose be considered as invariable from year to year, so that 
the two actions above mentioned show that the radiant absorbent 
and expansive powers of aqueous vapour combine to lessen the rela- 
tive amount of heat retained by the earth, during both exceptionally 
high and exceptionally low temperatures, 7. ¢., during a period 
of large thermometric range, and consequently to diminish the mean 
temperature. 
There may be and probably are other con-causes of this effect, but 
the one we have assigned is certainly a real and efficient factor 
in producing the apparently anomalous result in question. 
In the discussion which followed the reading of Mr. Clark’s 
paper, Mr. Geo. E. Shaw, Mr. J. G. Mowat, Dr. Jos. Workman, 
and Mr. J. M. Buchan took part. 
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